Are we talking about Head Start or the new elementary-school based Pre-K for kids who have IEPs? Because with the former, I agree somewhat with Dunn. Academically, Head Start has been proven time and time again to not have lasting effects. Sending your kid to Head Start at 4 isn't going to make him/her a genius, nor will they necessarily be "above average" by the time they're in older elementary grades.
Of course, that may have more to do with the elementary grades themselves than Head Start.
However, the new elementary based pre-k that is available through the Department of Education for kids who have IEPs (special needs) is a little different than the typical head start or pre-k class. The kids in THAT particular Pre-K program are learning a host of social, vocational and adaptable skills that will prepare them for being in a mainstreamed, regular classroom environment when they're ready for Kindergarten. Autistic kids and other kids with special needs are benefiting greatly from the speech/language, social and occupational therapy that they're receiving in the classroom environment starting at 3 and 4. It's another form of early intervention which is, as far as I can tell, really succeeding.
So yeah, it just depends on what Dunn is specifically talking about.